The present invention relates generally to a carrier for electrophotography, a method for producing the same carrier, and a developing agent for electrophotography using the same. More particularly, it relates to a carrier for electrophotography used in development of an electrostatic latent image in an image formation method utilizing electrophotography, a method for producing the same carrier, and to a developing agent for electrophotography using the same.
Hitherto known as an electrostatic latent image development method for electrophotography is a two-component developing method, which allows frictional electrification of a toner, transportation of a developing agent, contact with an electrostatic latent image, and development by mixing an insulating non-magnetic toner and magnetic carrier particles together.
The particulate carrier used in such two-component developing method is usually produced by coating a magnetic carrier core material with an appropriate material in order to prevent filming a toner onto the surface of the carrier, to form a uniform surface of the carrier, to elongate the lifetime of the developing agent, to prevent damage or friction by a carrier of a photoconductor (sensitizer), and to control electrification polarity or amount of electrification, and for other purposes.
Conventional resin-coated carriers are not, however, satisfactory in durability because the coat is easily exfoliated (delaminated) by shock or other reasons such as stirring applied when used.
For example, in Japanese Patent Laid-open Nos. Sho 64-33562 and Sho 64-33563, carriers for developing electrostatic image, which comprises a core material provided thereon in order by using a supiller coater, an intermediate resin layer containing a styrene resin, a polyolefinic resin and the like, and a layer for controlling triboelectric chargeability containing a fluorinated alkyl (meth)acrylate. However, the delamination easily occurs between the intermediate resin layer and the layer for controlling triboelectric chargeability. Also, when the polyolefinic resin is used as the material for the intermediate resin layer, it is difficult to form firmly on the surface of the core material by using a supiller coater. Therefor, the delamination easily occurs between the intermediate resin layer and the surface of a core material particle, too.
To solve this problem, the applicants have developed and proposed a method to form a polyolefin-based resin coat by directly carrying out polymerization of an polyolefin-based monomer on carrier core material particles of ferrite or other materials as described e.g. in Japanese Patent Laid-open No. Hei2-187771. A polyolefin-based resin-coated carrier obtained by this methods has the strong adhesion between a core material particle and a polyolefin-based coated resin layer. This is because the polyolefin-based resin layer is formed by directly polymerization method on the surface of a core material particle which is porous, and a part of such layer penetrates into the core material as shown in FIGS. 8-11. Also, the polyolefin-based resin-coated carrier does not give any deterioration in the quality of image, and is excellent in durability and spent-resistance even if copying is repeated continuously for a long time.
Here, FIGS. 8 and 9 respectively show the sectional photo-views of a covering layer of carrier A which magnetic carrier core material is eliminated and which is encapsulated with an epoxy resin under the reduced pressure condition in order to observe by using a Scanning Electron Microscope (hereinafter SEM). Thus, the inside part of the covering layer which correspond to the magnetic carrier core material was filled with an epoxy resin.
FIGS. 10 and 11 similarly show the sectional photo-views of a covering layer of carrier A which core material particle is eliminated and which is encapsulated with an epoxy resin in order to observe by using a SEM. However, such encapsulation was carried out at the normal pressure condition, not the reduced pressure condition. Thus, the inside part of the covering layer which correspond to the magnetic carrier core was not filled with an epoxy resin.
On the other hand, however, this polyolefin-based resin-coated carrier is not adequately satisfactory with respect to free controllability of the electrification polarity or the amount of electrification.
Therefore, a polyolefin-based resin-coated carrier having a covering layer which is formed by directly polymerization method in the condition that a particle having an ability to control electric charge is added, have been proposed in Japanese Patent Laid-open No. Hei 3-208060. However, there is a problem that the particle having an ability to control an electric charge in the direct polymerization, remarkably reduces the polymerization activity of a catalyst and the forming of the covering layer is inhibited. Also, even if the direct polymerization is carried out in the low concentration of the particles having an ability to control an electric charge, they are not dispersed uniformly on the surface of the carrier and the controlling ability of the electric charge is quite low. Moreover, there is a problem that the controlling ability of the electric charge is greatly varied after using for a long time.
On the contrary, a carrier having a polyethylene type covering layer which is formed by directly polymerization and in which a particle having an ability to control an electric charge is added onto the uneven surface by a supiller boater, have been proposed in Japanese Patent Laid-open No. Hei 6-266168. However, a particle having an ability to control an electric charge is easily dropped off from the surface of the covering layer since it is not fixed to the surface. Also, the controlling ability of the electric charge is low since a thickness of the particle layer having an ability to control an electric charge is not adequate.
This invention addresses to the above-mentioned problems, i.e. the purpose of the present invention is to provide a carrier for electrophotography, a method for producing the same carrier and a developing agent for electrophotography using the carrier, which allows free control of the electrification polarity and the amount of electrification, taking advantage of an excellent property of the carrier having a polyethylene resin covering layer.
In order to achieve the above-mentioned purpose, namely, to have a free control ability (electrostatic charging ability) of the electric charge polarity or the amount of electrification an excellent durability, as well as an excellent durability, the present invention provides a carrier for electrophotography having the following features as gists, a method for producing the same carrier, and a developing agent for electrophotography using the carrier.
[1] A carrier for electrophotography having a magnetic carrier core material and a polyethylene covering layer composed of a polyethylene resin having a weight-average molecular weight of 50,000 or more,
wherein the polyethylene covering layer is a layer formed by directly polymerizing an ethylene monomer on the surface of the catalytic treated carrier core material and
a particle layer having an ability to control an electric charge and having a thickness of 0.01-2 xcexcm, is formed on the surface of the polyethylene covering layer.
[2] The above carrier for electrophotography, wherein the particle having an ability to control an electric charge is fixed onto the polyethylene resin layer by a mechanical impact or a thermal fixation with heating.
[3] The above carrier for electrophotography, wherein an electroconductive layer is formed onto the surface of said carrier core material prior to forming the polyethylene covering layer.
[4] The above carrier for electrophotography, wherein the electroconductive layer is a layer formed by directly polymerizing method.
[5] A carrier for electrophotography having a magnetic carrier core material and a polyethylene covering layer composed of a polyethylene resin having a weight-average molecular weight of 50,000 or more,
wherein the polyethylene covering layer is a layer formed by directly polymerizing an ethylene monomer on the surface of the catalytic treated carrier core material and
a resin layer having an ability to control an electric charge and having a thickness of 0.01-2 xcexcm, is formed on the surface of the polyethylene covering layer and a part of the layer penetrates into the polyethylene covering layer.
[6] The above carrier for electrophotography, wherein an electroconductive layer is a layer formed onto the surface of said carrier core material prior to forming a polyethylene covering layer.
[7] The above carrier for electrophotography, wherein the electroconductive layer is a layer formed by directly polymerizing method.
[8] A method for producing a carrier for electrophotography having a magnetic carrier core material and a polyethylene covering layer composed of a polyethylene resin having a weight-average molecular weight of 50,000 or more, comprises
a step for forming the polyethylene covering layer by directly polymerizing an ethylene monomer on the surface of the catalytic treated carrier core material,
a step for attaching a particle having an ability to control an electric charge to the surface of the polyethylene covering layer,
and a step for forming a particle layer having an ability to control an electric charge and having a thickness of 0.01-2 xcexcm by a mechanical impact or a thermal fixation with heating.
[9] The method for producing the above carrier, further comprises a step for forming an electroconductive layer onto the surface of said carrier core material prior to forming a polyethylene covering layer.
[10] The method for producing the above carrier, further comprises a step for forming an electroconductive layer by directly polymerizing method.
[11] A method for producing a carrier for electrophotography having a magnetic carrier core material and a polyethylene covering layer composed of a polyethylene resin having a weight-average molecular weight of 50,000 or more, comprises
a step for forming the polyethylene covering layer having an uneven surface by directly polymerizing an ethylene monomer on the surface of the catalytic treated carrier core material,
a step for coating a solution including a resin having an ability to control an electric charge onto the surface of the polyethylene covering layer by using a wet-treatment, and
a step for forming a resin layer having an ability to control an electric charge and having a thickness of 0.01-2 xcexcm in the manner that a part of a resin layer penetrates the polyethylene covering layer by heating.
[12] The method for producing the above carrier, further comprises a step for forming an electroconductive layer onto the surface of said carrier core material prior to forming a polyethylene covering layer.
[13] The method for producing the above carrier, further comprises a step for forming an electroconductive layer by directly polymerizing method.
[14] A developing agent for electrophotography comprising the above-mentioned [1] or [5] carrier for electrophotography, and a toner mixed with said carrier at a ratio of 2-20 wt. %.